Visual evoked oscillatory responses of the human optic tract

Optic tract oscillatory responses directly recorded during posteroventral p allidotomy were investigated to reveal their features with respect to extra cranially recorded visual oscillations and to clarify their contributions t o scalp-recorded or far-field visual evoked potentials. Oscillatory respons es of the optic tract consisting of early and subsequent late oscillations were recorded in all patients. Early oscillations consist of five negative and positive peaks, and late oscillations consist of two to four negative a nd positive peaks. The frequency of the first peak of early oscillations (1 03.0 +/- 9.2 Hz, n = 14) was significantly lower than that of others (t tes t, P < 0.006), but there were no significant differences among other peaks (t test, P > 0.4). This difference was not observed among peaks of late osc illations (t test, P > 0.3). As a whole, the frequency of early oscillation s (123.9 +/- 16.7 Hz, n = 70) was significantly higher than that of late os cillations (66.3 +/- 13.7 Hz, n = 41) (t test, P = 0.0001). Intracerebral r ecording of the optic responses with the same band filter of scalp-recorded visual evoked potentials (10 Hz to 1 KHz) showed prominent negative (No) a nd positive (Po) waves, whereas responses obtained over the scalp at Oz and Cz consisted of negative (NI), positive (PI), negative (NII), and positive (PII) waves. Comparisons between No and PI and Po and NII showed overall p hase reversal relations between them, but there were significantly differen t peak latencies between them (t test, P < 0.001) except that between Po (1 16.7 +/- 11.7 msec, n = 5) and NII of Cz recording (118.4 +/- 9.4 msec, n = 5) (t test, P < 0.3). Our study in conjunction with other studies on visua l oscillations revealed that a relatively constant frequency of oscillation s is traveling in the subcortical visual system and is probably playing an important role in generating stationary or fixed peaks of the far-field pot entials of visual evoked potentials.